1. Field of the invention
The present invention relates to ethylene-vinyl alcohol copolymer composite fibers which are highly thermostable and can hence be used for clothing use.
ln particular, the present invention relates to a technique for obtaining a composite fiber comprising a component (A) of a saponified product of an ethylene-vinyl acetate copolymer and a component (B) of a thermoplastic polymer, which has excellent thermal stability so that said fiber or fabrics containing said fiber do not cause sticking or adhesion by component A by dry heat treatment or hot water treatment. The present invention also relates to a technique for preventing said composite fiber from coloring upon acetalization treatment. Further the present invention relates to a technique for preventing dyed articles comprising said fiber from discoloration upon heating after the acetalization treatment. Still further the present invention relates to a technique for dyeing articles comprising said fiber without causing the died articles to shrink or deteriorate and without impairing the hand and appearance of the articles. 2. Description of the prior art
Composite fibers comprising a saponified product of ethylene-vinyl acetate copolymer and a hydrophobic thermoplastic resin such as polyesters, polypropylene or polyamides are disclosed in for example Japanese patent publication Nos. 5846/1981 and 1372/1980.
Ethylene-vinyl alcohol copolymer fibers have, thanks to the hydroxyl groups contained in the molecules, superior features such as hydrophilic property, soil-resistant property and antistatic property as compared to conventional melt-spun synthetic fibers. However, they have drawbacks of 10 inferior thermal stability against high-temperature hot water, steam or the like because of their low melting point and softening temperature. The above-cited patents disclose a technique of providing a composite fiber comprising an ethylene-vinyl alcohol copolymer and a thermoplastic polymer having a thermal stability higher than the ethylene-vinyl alcohol copolymer, thereby providing the fiber with dimensional stability and the like. The fibers obtained by the techique however still have drawbacks of causing part of the ethylene-vinyl alcohol copolymer component exposed on the fiber surface to soften or slightly stick together to stiffen the hand or impair the appearance when dyed under high-temperature and high-pressure conditions or heated with a steam iron at sewing or on occasions during use. Then, for the purpose of dyeing the fiber without generating such trouble, the dyeing temperaure must be lowered to 90.degree. C. or below; and a dyeing at a temperature above this Would cause the ethylene-vinyl alcohol copolymer component to soften and fuse so that the desired product cannot be obtained. On the other hand, the other component of the composite fiber cannot sufficiently be dyed at such low temperature of 90.degree. C. or below. As a result, the fiber has no appropriate dyeing temperature range to dye the both components, thus having no dyeability. Furthermore, fabrics containing the fiber still have problems unsolved of generating a significant change in the appearance by ironing at sewing or on occasions during use. It is thought that such fatal problems have made the composite fiber of this type commercially unsuccessful.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a highly thermostable composite fiber comprising a component of a saponified product of an ethylene-vinyl acetate copolymer, which does not cause sticking or fusion of the component at high temperatures.
Another object of the present invention is to provide a process for producing such composite fiber.
Still another object of the present invention is to provide a method of treatment for the above composite fiber, which does not cause coloring at acetalization, discoloration of the dyed articles after the acetalization, or shrinkage or deterioration at dyeing.
Thus, the present invention provides a composite fiber comprising a component (A) of a saponified product of an ethylene-vinyl acetate copolymer having an ethylene content of 30 to 70 mol % and a component (B) of a thermoplastic polymer, said component A being exposed on at least part of the surface of said composite fiber and acetalized with a compound represented by the following formula [I] an said component A having a melting point satisfying the following formula [II] EQU OCH--C.sub.n H.sub.2n --CHO [I]
wherein n is 0 or an integer of 1 to 10, EQU -1.524.times.(Et%)+234&lt;Ma [II]
where
Et%=ethylene content in component A (mol %]and
Ma=melting point of component A (.C); and, more preferably, a composite fiber as defined above wherein residual non-crosslinked aldehyde groups of said compound have, after the acetalization reaction, been formed by action of NaHSO.sub.3 into --C.sub.n H.sub.2n CHO.NaHSo.sub.3, or oxidized into groups of carboxylic acid or a salt thereof.
Further the present invention provides a process for producing an ethylene-vinyl alcohol copolymer composite fiber, which comprises acetalizing:
a composite fiber comprising a component (A) of a saponified product of an ethylene-vinyl acetate copolymer having an ethylene content of 30 to 70 mol % and a component (B) of a thermoplastic polymer, said component A being exposed on at least part of the surface of said fiber, in the form of an aggregate of cut fibers, a yarn or a fabric,
at a temperature, T, of 15 to 135.degree. C. with a solution containing a strong acid and a compound of the above formula [I] in a concentration, N, of 0.05 to 2 normals and in a concentration, C, of 0.002 to 5 moles/1 respectively, said T, N and C at the same time satisfying the following relationship [III] EQU N.ltoreq.0.548-0.576.times.log C-6.3.times.10.sup.-3 .times. T [III]
where
N=concentration of strong acid (normals),
C=concentration of dialdehyde (moles/l) and
T=acetalization temperature (.degree.C);
said acetalization treatment being most preferably conducted with said acetalization solution further containing at least 5 g/1 of the salt of a strong acid and a strong base; and, more preferably, a process which comprises first heat-treating the above-described ethylene-vinyl alcohol copolymer composite fiber, in the form of an aggregate of cut fibers, a yarn or a fabric, at a temperature above 100.degree. C. and below the melting point of component A and then acetalizing the thus heat-treated fiber in the above-described way.
Still further the present invention provides a process for treating said composite fiber with a solution or dispersion of NaHSO.sub.3 before its exposure to a temperature above 140.degree. C., after acetalization and before or after dyeing; or treating said composite fiber with an oxidizing agent before its exposure to a temperature above 140.degree. C., after acetalization and before or after dyeing.
The present invention still further provides a method for dyeing the composite fiber after being acetalized, which comprises dyeing the fiber with an aqueous dyeing bath at 95.degree. C. or above, said dyeing bath containing at least 5 g/l of the salt of a strong acid and strong base, at least 10 g/l of boric acid or at least 1 g/l of the base of a strong acid and a strong base together with at least 5 g/l of boric acid.